Ab initio study of the structural response to magnetic disorder and van der Waals interactions in FeSe

The electronic structure in unconventional superconductors holds a key to understanding the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial results regarding the importance of the ${k}_{z}$ dependence of the electronic dispersion, the gap structure, and the pairing mechanisms. Here, we use density functional theory to investigate the underlying structural properties in combination with a sophisticated real-space treatment of magnetic disorder for the prototype system FeSe. Our calculations demonstrate that interlayer and intralayer interactions need to be considered and that charge-driven van der Waals interactions between Se atoms instead of magnetic coupling effects drive the interlayer binding. The methodological advances and physical insights are important for upcoming investigations of the three-dimensional effects, including nontrivial topology, of $\mathrm{Fe}{\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ and $\mathrm{Fe}{\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ systems.